1. Field of the Invention
This invention relates to wireless communications and, more particularly, to a rate feedback and adjustment system.
2. Description of Related Art
Wireless communications systems include conventional cellular communication systems which comprise a number of cell sites or base stations, geographically distributed to support transmission and receipt of communication signals to and from wireless units which may actually be stationary or fixed. Each cell site handles voice communications over a particular region called a cell, and the overall coverage area for the cellular communication system is defined by the union of cells for all of the cell sites, where the coverage areas for nearby cell sites overlap to some degree to ensure (if possible) contiguous communications coverage within the outer boundaries of the system's coverage area.
When active, a wireless unit receives signals from at least one base station or cell site over a forward link or downlink and transmits signals to (at least) one cell site or base station over a reverse link or uplink. There are many different schemes for defining wireless links or channels for a cellular communication system, including TDMA (time-division multiple access), FDMA (frequency-division multiple access), and CDMA (code-division multiple access) schemes. In CDMA communications, different wireless channels are distinguished by different codes or sequences that are used to encode different information streams, which may then be modulated at one or more different carrier frequencies for simultaneous transmission. A receiver can recover a particular stream from a received signal using the appropriate code or sequence to decode the received signal.
Due to the delay-intolerant nature of voice communication, wireless units in conventional cellular systems transmit and receive over dedicated links between a wireless unit and a base station. Generally, each active wireless unit requires the assignment of a dedicated link on the forward link and a dedicated link on the reverse link. Traditional data applications are typically bursty and, unlike voice communications, relatively delay tolerant. As such, using dedicated links to transmit data is an inefficient use of network resources. Wireless communication systems are evolving that will support a variety of data services, such as wireless web browsing.
In the well known data only evolution of the third generation CDMA standard (hereinafter referred to as 3G-1x EVDO), voice and data services are provided using separate frequency carriers. That is, the voice and data signals are transmitted over separate forward links defined by different frequency carriers. Data is transmitted over a time division multiplexed carrier at fixed data transmit powers but at variable data rates. Specifically, measured signal to interference ratio (SIR) at the receiver is used to determine a data rate which can be supported by the receiver. Typically, the determined data rate corresponds to a maximum data rate at which a minimum level of quality of service can be achieved at the mobile-telephone. Higher measured SIR translates into higher data rates, wherein higher data rates involve higher order modulation and weaker coding than lower data rates.
To improve system throughput, the system allows the wireless unit with the best channel, and thereby the highest rate, to transmit ahead of wireless units with comparatively low channel quality. 3G-1x EVDO uses a fast rate adaptation mechanism whereby the wireless unit performs the rate calculation at every slot using measurements of a pilot signal broadcast from the base station and reports back the rate at which it is going to receive data from the base station at every slot. In a symmetrical fashion, the base station receives a reported rate at every slot and sends data at the reported rate at every slot. The symmetrical rates for rate calculation and rate adaptation in 3G-1x EVDO disadvantageously requires large processing overhead for rate calculation and prediction every slot. Additionally, large transmission overhead is required to feedback the rate every slot.
In the Universal Mobile Telecommunications System (UMTS), wireless units communicate with a base station over dedicated channels. To provide efficient wireless data communications, UMTS uses a downlink shared channel which can be shared by a plurality of wireless units to receive data. To improve system throughput, the system provides the wireless unit with the best reported rate access to the shared channel. To alleviate the processing and transmission overhead problems associated with calculating and reporting the rate every slot, rate adaptation systems have been proposed that report the rate every 5 to 8 slots and in symmetrical fashion, the rate is adapted by the system every 5 to 8 slots.